1. Field of the Invention
This invention relates to solid state transmit/receive switches and, in particular, to transmit/receive switching circuits for radio transceivers. Accordingly, it is a general object of this invention to provide new and improved devices of such character.
A new and improved transmit/receive switching circuit in accordance with this invention provides an extremely reliable means for switching between transmit and receive (in a radio transceiver), requiring no external source of high voltage or current.
Further, a new and improved transmit/receive switching circuit in accordance with the invention provides automatic switching from a receive mode to a transmit mode when a significant voltage appears on the antenna line, whereby the receiver is protected against lightning, inadvertent transmitter operation, and electromagnetic pulses.
2. Description of the Prior Art
A common transmit/receive switch is, effectively, a single pole double throw device that connects the antenna either to the transmitter or to the receiver, as desired. Disadvantageously, the relay types in the prior art suffered from speed and reliability problems, while solid state versions often suffered from loss, especially in transmitting and often resulted in poor receiver isolation. Harmonic generation, occurring during transmission, provided the problem of off frequency received signals.
Solid state transmit/receive switches commonly utilized PIN diodes. PIN diodes, as is well known, is a diode that consists of a silicon wafer containing nearly equal P type and N type impurities with additional P type impurities diffused from one side and additional N type impurities diffused from the other side. PIN diodes differ from ordinary diodes in that the diode action is slow compared to the carrier frequency, with the result that a small reverse dc voltage (with the diode nonconducting) or a small forward dc current (with the diode conducting) controls much higher radio frequency voltages and currents. They work best at the higher rf frequencies where the speed of the diode is much slower than the carrier cycle time. Low frequency PIN diodes are available that are useful at medium frequencies, such as the Unitrode UM7010. All PIN diodes have low capacitance, which is desirable, but they require substantial forward current (in the hundreds of milliamperes) to attain a low forward resistance.
For many years, the function of a transmit/receive switch, for switching between transmit and receive modes, utilizing no external source of high voltage or current, was performed with a relay. More recently, PIN diodes have been utilized requiring external control voltages and currents. The function of providing automatic switching to transmit when significant voltages appear on the antenna line was not provided in the relay systems of the prior art, and they probably were not, to the knowledge of the applicant, utilized in PIN diode switches because circuits to change the control voltages would have to be added.
The relay circuits of the prior art tended to be unreliable and were large, slow and power consuming. The PIN diode systems of the prior art were lossy, generated harmonics and spurious frequency products from two or more signals and required an external supply of high voltage (with non-conducting diodes) and high currents (with conducting diodes). Especially at the lower frequencies, the voltage and current requirements became objectionably large for a good performing switch. The PIN diode switches often included a quarter wave transmission line that could be large at low frequencies which limited the bandwidth.